Load balancing is important to use hardware and network resources efficiently. There are some load balancing technologies (e.g., DNS Round Robin, LVS (Linux Virtual Server), Apache) that can distribute clients' traffic between several real servers (RSs). The problem with these specific techniques is that once a connection is distributed to one server, the connection cannot be transferred to other RS(s). If hardware and network load are stabile, one can set an appropriate threshold for the purpose of load balancing and redistributing the existing load is not so difficult. However, in general, because both hardware and network resources are shared among many applications and services, the load of which can change quite dynamically over time. The load balancing (or equivalently resource allocation) decisions done at an earlier point in time might be wrong for the current load. If load redistribution to achieve better resource management is delayed, users may not receive their expected quality of service. To set a threshold with a large margin of error is the easiest way of sustaining service quality, but it will decrease resource utilization.
Virtualization gives flexibility for resource management. VMotion and Xen's Live Migration enable moving guest OS states from one physical location to another. Since all guest OS related states are transferred, all connection can be preserved after the migration. These technologies alleviate quality degradation and resource under utilization problems because all connections can be moved to a better place near real-time after the need of load redistribution arises (e.g., load on a guest OS exceeds a specified or dynamically computed threshold).
There are still several limitations for these virtualization technologies. First, these migrations can only be done inside the same LAN, because if an IP address is changed during the migration, ongoing connections are terminated and cannot be transferred to new location. Moreover, the entire traffic is transferred to new location, which will cause a resource shortage problem at the new location too.